Geosynchronous (“GEO”) spacecraft are frequently used for applications in which high-accuracy orbital position knowledge is necessary. For example, high accuracy pointing is needed for spacecraft that include celestial-inertial attitude determination systems using star trackers. Also, high accuracy orbit position information is needed for remote sensing applications, where precise instrument image navigation and registration (“INR”) is required.
Such missions may require orbit position knowledge of 75 meter per axis or better, at all times including during and after thruster orbit control maneuvers. Furthermore, future spacecraft must operate autonomously, without ground commanding for extended periods, even during periods that include thruster maneuvers.
One approach to providing high accuracy orbit position information uses NAVSTAR Global Positioning System (“GPS”) signals to determine the position and velocity of a spacecraft. In this approach, signals from four or more GPS satellites are used to compute the position and velocity of a spacecraft. However, because GEO spacecraft are well above the altitude of the GPS constellation, and the GPS satellite antennas are pointed toward the Earth, signals from four GPS satellites may not be continuously available to a GPS receiver in GEO. Therefore, an Extended Kalman Filter (“EKF”) may be implemented to provide a continuous estimate of the position and velocity of the spacecraft, even when less than four GPS signals are available
Using an EKF, such GPS-at-GEO systems can provide reasonably accurate performance, assuming they include suitably, designed antennas and receivers that can acquire and track the low-level spillover signals from the GPS satellites. To improve their performance, these systems may include a precision orbit model for modeling perturbation accelerations, such as the gravity effects due to the Earth, Sun, and Moon, as well as the solar pressure force. While these perturbations can be reasonably well modeled, difficulty occurs when thrusters are fired to change the spacecraft velocity or to adjust the momentum of the spacecraft reaction wheels. In these cases, there will be significant uncertainty in the orbital effect of the thruster firing due to, for example, thruster performance variability, plume impingement, and other propulsion and spacecraft uncertainties. In the presence of these uncertainties, it may not be possible to meet precision navigation requirements during or immediately following a maneuver when few GPS satellites are visible.
Thus, satellites using this approach rely upon ground-based calibration to account for the effects of thruster firing. The updated information may be periodically supplied to the spacecraft, to improve navigation performance during and after each maneuver. The drawback of this approach is that, because the calibration procedure is implemented on the ground, thruster model updates cannot be made during periods where the spacecraft must operate autonomously. Furthermore, the calibration procedures are labor intensive, increasing spacecraft operational cost. Also, calibrations must be maintained for all of the thruster sets, and care must be taken to ensure that correct information is uploaded to avoid a mistake that might disrupt mission operations. Finally, existing ground-based orbit determination calibration procedures are too computationally intensive to be suitable for implementation onboard the spacecraft.